atoms in space makes the 1,4-diene more accessible. In other properties of interest, Janoschek calculated a heat of formation of 93.7 kcal per mole, compared with 22.0 kcal per mole for benzene. This higher energy content comes from sacrificing benzene's 1,3,5-cyclohexatriene stability, the bending of the C=C=C angle to 132.4° from 180°, and changing C-6 hybrid orbitals to sp3 from sp2. And the isobenzene is chiral. The two hydrogens on either end of the C=C=C sequence are staggered so that two nonsuperimposable mirror images are possible. An analog of the Wiirzburg isonaphthalene was made earlier. Organic chemistry professor Bernard Miller of the University of Massachusetts, Amherst, reported trapping a gem-dimethyl derivative in 1987. The gem-dimethyl grouping prevented reorganization of the allene sequence. And in 1990, organic chemistry professor Richard P. Johnson of the University of New Hampshire, Durham, trapped 1,2,3cyclohexatriene and cyclohexen-3-yne. Stephen Stinson
Silicone implants studied with MRI Magnetic resonance imaging (MRI) studies of implanted silicone devices to assess how they change with time have been made by radiologists at Massachusetts General Hospital and Harvard Medical School, Charlestown. Although the studies were in rats, not humans, and maximum elapsed time was six months, rather than decades, results to date indicate that a combination of in vivo imaging, localized spectroscopy, and in vitro multinuclear solidstate spectroscopy is useful for the study of age-induced changes in implant devices. The method may be used in some of the studies called for by the Food & Drug Administration to gauge longterm safety of silicone implants. Such implants are used for reconstruction of breasts as well as damaged facial features and finger joints and for controlled release of drugs. The study group's findings were described by Bettina Pfleiderer at the national meeting of the American Chemical Society last month in San Francisco.
Conventional spin echo image (top) shows both lipid and silicone protons, implant (arrow) is indistinguishable. Special pulse-sequence image (bottom) shows only the implant. Working with John Moore, Jerome L. Ackerman, and Leoncio Garrido, Pfleiderer implanted silicone rubber bags, 25 mm on a side, filled with silicone oil, into the lower backs of rats. The workers then monitored the condition of the implants using a pulse-sequence MRI technique that would image the protons of poly(dimethylsiloxane) but suppress those of lipids or tissue water. Whole-body imaging resolution was adequate to detect cracks or rupture in implants, and there were none during the six months of study. Removal of implants after three months showed no changes in solidstate NMR images and no migration of silicone materials into surrounding tissue. Implants removed after six months were found to have absorbed minute amounts of lipids from surrounding tissue, and the silicone rubber had decreased rupture resistance. The molecular structure of the poly(dimethylsiloxane) resin seemed unchanged, however. And again no silicone was found in surrounding tissue. The group plans to continue their research with nine- and 12-month studies. Stephen Stinson
